This invention relates to the fabrication of electrode and separator components for batteries.
Thermal batteries are a type of battery characterized by long shelf-life (>20 years) even under extreme environmental conditions. They exhibit high functional reliability even after prolonged storage and possess high power capability. A typical thermal battery comprises an alkali alloy, such as a lithium alloy, or alkaline earth metal anode, a salt electrolyte, a metal salt, such as a metal chromate or metal sulfide cathode, and a heat source that is usually positioned between the cells. Cells are stacked in bipolar configuration and thermal insulation is positioned around the circumference and at both ends of the cell stack. The battery case is hermetically sealed. An energy impulse from some external source activates the heat source (often pyrotechnic materials) to melt the electrolyte. The battery then becomes ionically conductive and produces high power for a short period of time, typically from a few seconds to an hour or so. A typical thermal battery operates over the temperature range of approximately 352 to 600° C.
The most common anode material is a lithium alloy such as Li(Si) or Li(Al), although calcium is also sometimes used. The electrolyte is typically a molten eutectic mix of lithium chloride and potassium chloride which has a melting point of 352° C. Some materials that can be used for cathodes in various batteries designed to operate at elevated temperature or at ambient temperature include calcium chromate, potassium chromate, potassium dichromate, lead chromate, metal oxides, metal sulfides, metal phosphates, metal oxyphosphates, poly-carbon monofluoride, carbon, carbon black, and graphite. The Li(Si)/FeS2 cell configuration has the advantages of operation under discharge conditions from open circuit to high current densities, tolerance to processing variation, and stability in extreme environments. Disadvantages of current designs of thermal batteries include low energy density, activated surface temperature of 230° C. or higher, and high cost.
The basic design and production methodology of thermal batteries was developed in the mid-1960s, and since then, there have been some changes in the anode, separator and cathode materials. However, the basic design and fabrication processes remain the same, and the use of these processes yield batteries that are often drastically overbuilt yet not robust.
A conventional thermal battery consists of a stainless steel can which contains a bipolar stack of cells consisting of discrete elements, including the cathode pellets, anode pellets, electrolyte pellets, heat pellets, stainless-steel current collectors, grafoil current collectors; and thermal insulation. The pellet geometry (surface area and thickness) is dictated by the pressing processes used to fabricate the pellets. A minimum pellet thickness is required to achieve mechanical integrity, and this thickness is typically far greater than that required to meet the electrical requirements of the battery. Currently available thermal batteries often contain 5-fold to 40-fold excess of active ingredients, and therefore they operate at a Coulombic efficiency of only 2.5 to 20%. Additionally, the excess material in the pellet, which facilitates the mechanical integrity of the pellet, also increases the pellet resistance, and thereby reduces the output power. In addition to the problems of power loss and low Coulombic efficiency, the excess mass of pellet-based components must be heated and maintained at temperature, so additional weight and volume penalties result from the need for excess heat powder, excess insulation, and a larger/heavier stainless steel container. Thus, advances in thermal battery performance have been limited by continued use of the pellet processing technology originally developed in the 1960s. Very significant improvements in energy density and power density could be made by eliminating the excess active materials in the battery components by making them thinner. A change from powder processing and pellet pressing to materials and processes that can produce thinner components can realize major improvements in battery performance.